Functional description

ST1S40

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DocID17928 Rev 5

5.1

Internal soft-start

The soft-start is essential to assure correct and safe startup of the step-down converter. It

avoids inrush current surge and causes the output voltage to increase monothonically.

from 0 V to 0.8 V in around 1 ms.

5.2

Error amplifier and control loop stability

The error amplifier compares the FB pin voltage with the internal 0.8 V reference and it

provides the error signal to be compared with the output of the current sense circuitry, that is

the high side Power MOSFET current. Comparing the output of the error amplifier and the

peak inductor current implements the peak current mode control loop.

The error amplifier is a transconductance amplifier (OTA). The uncompensated

characteristics are listed inTable 5.

Table 5. Error amplifier characteristics

The ST1S40 device embeds the compensation network that assures the stability of the loop

in the whole operating range. All the tools needed to check the loop stability are shown

below.

Parameter

Value

DC Gain

95 dB

Gm

251 µA/V

Ro

240 M

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